In patent document FR-A 82 14034, an electronic head was described which comprised a laser source, a beam expander and a beam splitter consisting of a parallel-sided semi-reflecting mirror, disposed at a 45.degree. angle to the expanded beam.
This provides a reflected beam, sent to a reference photocell, and a transmitted beam which is directed to a measuring photocell after reflecting off the sample thin layer and again reflecting off the splitter.
Such a device presents the disadvantage of utilizing only a fraction of the light emitted by the laser source, only half of the laser light being transmitted through the mirror and only half of the beam reflected from the layer being reflected toward the measuring cell, so that only a quarter of the light emitter by the laser is utilized for measurement purposes.
One object of the present invention is to provide a device using virtually all of the light emitted by the laser, thus increasing the sensitivity of the device.
Another object of the invention is to combine two devices of the type just described, in order to provide a two-station thin layer thickness measuring device.